I need to add a gratuity to my site when user add a product to a cart. The function is the same to the ioby.org site. Each time the cart is updated we will update the gratuity on the cart. I have read the solution for this function from newsignature who developed this site but it's hard for me to using rules to do it. Anyone can help me? Thanks so much!
Here is the solution from newsignature: